1. Field of the Invention
The present invention relates to an electric steering system.
2. Description of the Related Art
As a steering system for a vehicle, an electric power steering system is known, in which a steering shaft coupled to a steering wheel is mechanically coupled to a steering mechanism for steering steered wheels of the vehicle, and an electric motor for assisting steering power is linked to the steering mechanism. Generally, an electric current for driving the motor is controlled in a manner such that the greater the steering input (such as steering torque) applied to the steering wheel by an operation of the driver, the greater the assistant steering power.
In an example of the electric power steering system, the current for driving the motor is controlled so as to apply damping to a steering shaft in accordance with a steering (rotation) speed of the steering wheel (see, for example, Japanese Unexamined Patent Application, First Publication No. H05-024546).
FIG. 11 is a diagram showing the structure of a conventional electric power steering system 1000 (i.e., an electric steering system). FIG. 2 is an enlarged sectional view showing a vicinity of a rack end of the electric steering system similar to the electric steering system 1000 shown in FIG. 11. When the steering mechanism employs a rack and pinion system, generally (see FIGS. 11 and 2) both heads of a rack (shaft) 8 protrude from open ends of a rack housing 21 of a steering gear box, and a tie rod 9 is coupled to a rack end 8b attached to each head of the rack 8. In this steering mechanism, when the steering wheel is rotated to the utmost limit, a stopper 13 provided on the inner side of the rack end 8b hits a fitting portion 22 of the rack housing 21, so that the rotation of the steering wheel is restricted and the rotation angle of the steering wheel reaches a maximum value.
In the steering mechanism having the above-described structure, when the rotation angle of the steering wheel reaches the maximum, the stopper 13 may be compressed and deformed between the rack end 8b and the fitting portion 22 of the rack housing 21. In such a case, the rack end 8b may hit the open end 21a of the rack housing 21, thereby generating a foreign sound. In addition, when the rack end 8b hits the open end 21a, an impact load may be applied on the rack end 8b, the open end 21a of the rack housing 21, a rack and pinion mechanism having a pinion 7 and a rack teeth of the rack 8 which are engaged with each other, or a bearing or a housing for supporting the rack and pinion mechanism.